Solid-core fiber waveguides are used to transport laser beams from a laser beam source to another device. FIG. 1 illustrates a typical solid-core fiber waveguide 100 of the prior art. Solid-core fiber waveguide 100 includes a core 120 and a concentric circular cladding ring 130. The core 120 extends along the solid-core fiber waveguide axis and has an index of refraction. The concentric annular cladding ring 130 surrounds core 120 and have a different index of refraction. Both the solid core 120 and concentric annular cladding ring 130 are formed by a single state of matter such as glass or other ceramic materials. The core 120 and concentric annular cladding ring 130 are encased by protective outer layer 110 such as polymer.
The core 120 and concentric annular cladding ring 130 of solid-core fiber waveguide 100 are comprised of a solid dielectric optical fiber. Though the solid-core fiber waveguide has many uses, the solid dielectric optical fiber waveguide is not suitable to transport certain laser beams. For example, laser beams with high energy, larger than 10 microjoules (μJ), and ultra-short pulse widths, less than 10 picoseconds (ps), cause laser irradiance that induces pulse distortion and optical damage to the waveguide material. The irradiance cannot be reduced past a fundamental limit by expanding a mode field area since multi-mode effects set in or the guiding mechanism (for example the index contrast) is too weak for practical transport.
Previous hollow-core fiber waveguides, such as hollow-core Bragg fiber and hollow-core resonant photonic bandgap fiber, have been used to transport low energy, low power ultra-short pulse width lasers. The previous fiber formats are inherently difficult to manufacture with determinism and difficult to scale the performance for high energy or high power short pulse laser compatibility.
Bragg fibers require exotic polymer and glass materials to achieve the high/low refractive index contrast bilayers that form the concentric rings that comprise the multi-layer dielectric mirror-based waveguide, and the materials must have matched thermal and glass transition properties for realistic fiber draw techniques. Moreover, the state-of-the-art hollow-core Bragg fiber preform fabrication techniques have poor dimensional repeatability compared to telecommunications grade fiber processes.
In general, resonant photonic bandgap fibers, e.g. photonic crystal fiber, utilize a glass lattice in the core of the fiber to form a forbidden zone for in-band wavelength light. The lattice must have very precise cell diameter and cell wall thickness to sustain the forbidden zone that enables waveguiding. In particular, there is an upper limit on these dimensions above which the resonant photonic bandgap, hence the waveguide, does not form. Scaling the hollow core diameter to handle ablative energy/power level laser beams imposes too high a mechanical load for the thin cell walls to support. Thus, state-of-the-art resonant photonic bandgap fiber has a hollow core limit of 20 micrometers (μm) in diameter. Moreover, even the minimal heating caused by interaction between the fiber guided modes and the thin cell walls results in catastrophic fiber damage for pulse energy of several microjoules or average power much less than a watt.
A waveguide is needed that is suitable for use with high energy high power ultra-short pulse width lasers.